Reddy Th Dhileep N, Mallik Bhabani S
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi-502285, Sangareddy, Telangana, India.
Phys Chem Chem Phys. 2020 Feb 14;22(6):3466-3480. doi: 10.1039/c9cp06796e. Epub 2020 Jan 27.
Microscopic structural and dynamic heterogeneities were investigated for three ionic liquids (ILs), namely tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), and tetrabutylammonium hydroxide (TBAH), by employing classical molecular dynamics (MD) simulations. Structural heterogeneity was explored through radial distribution functions, domain analysis from Voronoi tessellation, spatial distribution functions, combined distribution functions, and structure factors. Radial distribution functions reveal that TEAH has a slightly different structure than TPAH and TBAH. The domain analysis shows that the three ILs have structural heterogeneity. The cations form continuous domains and the anions form discreet domains. The anions are trapped in the voids formed by the cationic domains. When the cation has sufficiently longer alkyl chains, the polar domains become discontinuous. The total structure factor reveals that a pre-peak is absent in TEAH due to a short alkyl chain. The cation head-anion partial structure factor further confirms this phenomenon. Polar and non-polar domains can be better distinguished in the ILs with longer alkyl chains. This distinctive structural behaviour leads to dynamic heterogeneity, which plays an important role in the applications related to ion transportation. To shed light on dynamic heterogeneity, we have calculated the mean square displacements, vibrational density of states, van Hove correlation, and non-Gaussian parameter. The diffusion of ions is lowest in TPAH among all the ILs. The dynamics are highly sluggish in these ILs. The peaks in the low frequency vibrational density of states undergo redshift with an increase in the alkyl chain length. The van Hove correlation functions and non-Gaussian parameter suggest the presence of dynamic heterogeneity in all three ILs. Dynamic heterogeneity is discussed through van Hove correlation functions and the non-Gaussian (NG) parameter was calculated to account for the deviation from Gaussian behaviour. The NG parameter suggests that TEAH has different dynamics than TPAH and TBAH.
通过采用经典分子动力学（MD）模拟，研究了三种离子液体（ILs），即氢氧化四乙铵（TEAH）、氢氧化四丙铵（TPAH）和氢氧化四丁铵（TBAH）的微观结构和动态非均质性。通过径向分布函数、基于Voronoi镶嵌的域分析、空间分布函数、联合分布函数和结构因子来探索结构非均质性。径向分布函数表明，TEAH的结构与TPAH和TBAH略有不同。域分析表明，这三种离子液体具有结构非均质性。阳离子形成连续域，阴离子形成离散域。阴离子被困在阳离子域形成的空隙中。当阳离子具有足够长的烷基链时，极性域变得不连续。总结构因子表明，由于烷基链短，TEAH中不存在预峰。阳离子头-阴离子部分结构因子进一步证实了这一现象。在具有较长烷基链的离子液体中，可以更好地区分极性和非极性域。这种独特的结构行为导致了动态非均质性，这在与离子传输相关的应用中起着重要作用。为了阐明动态非均质性，我们计算了均方位移、振动态密度、范霍夫关联和非高斯参数。在所有离子液体中，TPAH中离子的扩散最低。这些离子液体中的动力学非常缓慢。低频振动态密度中的峰随着烷基链长度的增加而发生红移。范霍夫关联函数和非高斯参数表明所有三种离子液体中都存在动态非均质性。通过范霍夫关联函数讨论了动态非均质性，并计算了非高斯（NG）参数以解释与高斯行为的偏差。NG参数表明TEAH的动力学与TPAH和TBAH不同。
